1. Field of the Invention
The present invention generally relates to a valve for injecting a sample into a flowing stream and, more particularly, to a two-position rotary valve for injecting a liquid sample into a high pressure stream of liquid by means of a conventional low pressure volumetric syringe.
2. Description of the Prior Art
In the field of liquid chromatography, the contents of a liquid sample, which consists of unknown quantities of compounds, is analyzed by injecting the sample in a stream of a suitable eluting solvent, which passes through the chromatographic column and therefrom to an appropriate detector. Typically, the solvent from an appropriate source is pumped to the column by a pump, e.g., a constant flow pump. Modern columns generate relatively high back pressures under normal operation, e.g., 2000 psi and above.
For many years the fixed loop sample injection valve has been used in high pressure liquid chromatography because of its convenience of use, reliability and precision. Its major disadvantages are that a considerable amount of sample is wasted in the process of sample loading to insure that the sample loop is completely filled. Also, sample size can be changed only by changing the loop size. Syringe injection methods have been employed to circumvent these problems. The use of an elastomeric septum to permit direct sample injection by means of a syringe has the disadvantages of incompatibility with many eluting solvents, limitation of operating pressures to approximately 1000 psi and the requirement of special syringes with high pressure capability. Septumless injection devices have been designed for use with syringes. However, these devices require stopping the solvent flow for a reasonably long period, which is undesirable, since it usually leads to problems in stabilizing the detector signal.
Thus, a need exists for a simple, yet highly reliable sample injecting device which eliminates the disadvantages of each of the above described methods, while maintaining the advantages of reliable high pressure capability, convenient use of conventional syringes, variability of sample size, minimal sample waste, solvent compatibility and uninterrupted or substantially uninterrupted solvent flow.